Porcine Reproductive & Respiratory Syndrome Virus. Porcine reproductive and respiratory syndrome virus (PRRSV) is a positive-strand RNA virus that belongs to the Arteriviridae family. The PRRSV genomic RNA is approximately 15 kb comprising multiple open reading frames (“ORFs”) encoding the RNA replicase (ORF1a and ORF1b), the glycoproteins GP2 to GP5, the integral membrane protein M, and the nucleocapsid protein N (ORFs 2 to 7). Exemplary strains are the North American and European PRRSV strains represented by the prototype VR-2332 and Lelystad virus (LV) strains, respectively (Nelsen et al., J. Virol. 73:270-280, 1999).
Porcine reproductive and respiratory syndrome. PRRSV and porcine parvovirus (PPV) are the most common viral causes of porcine reproductive failure (Mengeling et al., Anim. Reprod. Sci. 60-61:199-210, 2000). PRRSV causes porcine reproductive and respiratory syndrome (“PRRS”; variously referred to as “mystery swine disease,” “swine infertility and respiratory syndrome,” “porcine epidemic abortion and respiratory syndrome,” abortus blauw” and “blue ear disease”), which is a major problem to the swine industry worldwide (Meng, Vet. Microbiol. 74:309-29, 2000). The respiratory form of the disease exhibits clinical signs which are most pronounced in piglets of 3-8 weeks in age, but are reported to occur in pigs of all ages in infected herds. The diseased piglets grow slowly, have roughened hair coats, respiratory distress (e.g., respiratory dyspnea or “thumping”) and increased mortality (up to about 80% pre-weaning mortality).
PRRS is associated with both gastrointestinal and systemic secondary infections (e.g., interstitial pneumonia, diarrhea, Salmonella choleraesuis, Streptococcus suis, Haemophilus parasuis, Actinobacillus pleuropneumoniae) that often overwhelm the host in advanced stages.
PRRS is also associated with ‘wasting syndrome.’ Pigs surviving PRRSV infection are often afflicted by retarded growth rates (i.e., “runt,” or “wasting” syndrome) and require additional time on feed before becoming large enough (if ever) for slaughter, producing particularly marked and commercially devastating effects.
Epidemiologically, PRRSV infection is a communicable disease that is both epidemic and endemic. It can spread like an epidemic in naive swine populations, but appears to endemically linger in affected populations (Blaha, Vet. Res. 31:77-83, 2000). A typical epidemic of PRRSV-induced reproductive failure is presented as a broad spectrum of clinical features including infertility, anorexia, delayed return to estrus, abortions, pre-mature births, late-term dead fetuses, stillborn pigs, weak-born pigs, and in its most severe form, sow death. There may also be an increase in the number of mummified fetuses in the later stages of an PRRSV epidemic (Mengeling et al., supra). The initial infection of sows may go unnoticed, or may manifest itself by an impaired condition or general malaise lasting up to a few days. For example, the sows may go “off feed,” and experience body temperatures either above or below normal. In the farrowing phase, the sows may exhibit depression, lethargy, pyrexia and occasional vomiting. In some affected herds, up to 75% of all piglets may be lost. The economic consequences of the disease are thus devastating.
PRRSV infects and replicates in porcine alveolar macrophages (PAMs) which are the cells of predilection in the natural host. Although, PAMs survive in culture for several days or even weeks, like other differentiated cells they eventually undergo senescence and die. Acquisition of unlimited growth in vitro is one of the characteristics that define cellular immortality.
The cell-line currently available for propagation of PRRSV in vitro is the green monkey kidney cell-line (and its derivatives), which has been patented (U.S. Pat. Nos. 5,476,778; 5,840,563; 5,846,805; 5,989,563; 6,042,830; US 2001 21383 and patents and patent applications derived therefrom, both U.S. non-U.S.).
There is, therefore, a pronounced need in the art for the development of alternative cell culture systems for propagation (e.g., in vitro and in vivo propagation) of PRRSV that would provide for preparation of virus and development of efficacious vaccines based thereon, or based on epitopes thereof.